Fire leads to flood all too often, but seldom with such tragic results as in Southern California early Tuesday. Flash floods and mudslides took at least 13 lives in Montecito, an unincorporated coastal area of about 9000 residents just east of Santa Barbara. Heavy overnight rains cascaded onto hillsides denuded by catastrophic fire in December, leading to the floods and mudslides. Update (10:15 am EST Wednesday): The Associated Press confirmed on Wednesday morning that the death toll had risen to 15, as reported by weather.com.

At least 25 other injuries were reported in Montecito. Dozens of residents were rescued, according to the Los Angeles Times. The toll may rise as searchers make their way into areas that were impossible to access late Tuesday. A smaller but still-damaging mudslide was reported in a Burbank neighborhood affected by the La Tuna Canyon Fire last September.

The damage in Montecito “looked like a World War I battlefield,” Santa Barbara sheriff Bill Brown told CNN. “"It was literally a carpet of mud and debris everywhere, with huge boulders, rocks, downed trees, power lines, wrecked cars…”

#CAstorm- With water levels receding, a vehicle is shown that was wrapped and tangled around a tree by the force of deadly flood waters and mudflow on Hot Springs Road in Montecito. In the background are homes that were completely destroyed. pic.twitter.com/w3lp7bokhf

A pathway to disaster, carved out by flame

Montecito had been under a flash flood watch since Monday afternoon, and residents were warned that “flash flooding and debris flows will be a particular threat in and below the recently burned areas.” There were widespread evacuations in and near the mammoth Thomas Fire burn scar. Some areas had been placed under mandatory evacuation orders, though not the hardest-hit parts of Montecito, reported the L.A. Times.

The storm that triggered Tuesday’s disaster was certainly a wet one for California, albeit not a record-breaker. San Francisco International Airport picked up 3.12” on Monday, which made it the fourth wettest calendar day in records that go back to 1945. The 3.15” in downtown San Francisco was the sixteenth-highest calendar day total since records began there in October 1849. These calendar-day totals happened to benefit from the storm’s Bay Area timing, as most of the rain fell snugly in the midnight-to-midnight window, according to WU weather historian Christopher Burt. “By no means did this rain event rank among what the 24-hour totals may have been during previous events,” said Burt.

For Southern California, rainfall amounts came in slightly under the forecast amounts issued late Sunday. The two-day totals of 2.52” in Santa Barbara and 1.49” in downtown Los Angeles were substantially less than two-day totals of 4.31” and 2.10” recorded just last year, in mid-February 2017.

However, the rainfall rates were very heavy—a classic risk factor for flash flooding—in and near the Thomas Fire burn scar, boosted by thunderstorms embedded within the rain areas. A total of 0.54” was reported in just five minutes at Montecito and 0.86” in 15 minutes at Carpenteria. "Any storm that has intensities greater than about 10 millimeters/hour (0.4"/hour) poses the risk of producing debris flows” when occuring over fire-scarred areas, says the U.S. Geological Survey.

How fire begets flood

NASA offers a concise explanation as to why burned hillsides are such a flood risk. Not only do the fires consume leaves and other ground cover that would otherwise absorb rainfall, but they can also inhibit the water-absorbing capacity of the soil itself. As explained by NASA:

“Plants and trees have numerous protective chemicals with which they coat their leaves to prevent water loss. Many of these substances are similar to wax. Vaporized by the heat from fires, these substances disperse into the air and then congeal over the soil surface when the fire begins to cool. Like the wax on your car, these substances coat the soil, causing water to bead up and run off quickly. In general, the greater the fire intensity and the longer the fire’s residence time, the more hydrophobic the soil becomes.” The Thomas Fire was an exceptionally intense and long-lasting fire, as noted by California Weather Blog.

The fatal mudslide in Montecito was actually south (downhill) from the area scorched by the Thomas Fire, according to Santa Barbara County Fire Department spokesman Mike Eliason. However, any rainfall and fire debris flowing off the burned areas upstream would have headed down the hillside toward Montecito. The NWS office in Riverton, WY, cautions: “A good rule of thumb is: ‘If you can look uphill from where you are and see a burnt-out area, you are at risk.’ ”

Still a relatively dry winter (up to now) in California

Even after the Monday/Tuesday deluge, California has yet to catch up to the precipitation amounts received by early January in an average year. The March-to-December period brought just 0.69” of rain to Los Angeles. That smashed the previous Mar-to-Dec. record of 1.24”, as reported by California Weather Blog. For the state’s entire South Coast Drainage climate region, the Oct-to-Dec. period was tied for the second driest in records going back to 1895, topped only by 1929 and matched only by 1903.

Statewide, the picture wasn’t quite so grim. Water captured in state snowpack was at 24% of average on January 3, reported the California Department of Natural Resources. In California’s boom-or-bust hydrologic regime, many winters are quite dry, while others are excessively wet, so it’s not all that unusual to be this far behind at this point in the water year (October 1-present).

Based on the GFS and ECMWF models, it will be at least early next week before we need to watch for California's next major statewide storm.

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Bob Henson

WU meteorologist Bob Henson, co-editor of Category 6, is the author of "Meteorology Today" and "The Thinking Person's Guide to Climate Change." Before joining WU, he was a longtime writer and editor at the University Corporation for Atmospheric Research in Boulder, CO.

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